Long-lived Single Excitons, Trions, and Biexcitons in CdSe/CdTe Type-Ⅱ Colloidal Quantum Wells

摘要

Light-harvesters with long-lived excited states are desired for efficient solar energy conversion systems.Many solar-to-fuel conversion reactions,such as H2 evolution and CO2 reduction,require multiple sequential electron transfer processes,which leads to a complicated situation that excited states involves not only excitons (electron-hole pairs) but also multi-excitons and charged excitons.While long-lived excitons can be obtained in various systems (e.g.,semiconductor nanocrystals),multi-excitons and charged excitons are typically shorted-lived due to nonradiative Auger recombination pathways whereby the recombination energy of an exciton is quickly transferred to the third carrier on a few to hundreds of picoseconds timescale.In this work,we report a study of excitons,trions (an exciton plus an additional charge),and biexcitons in CdSe/CdTe colloidal quantum wells or nanoplatelets.The type-Ⅱ band alignment effectively separates electrons and holes in space,leading to a single exciton lifetime of 340 ns which is ～2 order of magnitudes longer than that in plane CdSe nanoplatelets.More importantly,the electron-hole separation also dramatically slows down Auger decay,giving rise to a trion lifetime of 70 ns and a biexciton lifetime of 11 ns,among the longest values ever reported for colloidal nanocrystals.The long-lived exciton,trion,and biexciton states,combined with the intrinsically strong light-absorption capability of two-dimensional systems,enable the CdSe/CdTe type-Ⅱ nanoplatelets as promising light harvesters for efficient solar-to-fuel conversion reactions.